1. Field of the Invention
The present invention relates to packaging of microwave monolithic integrated circuits and more particularly concerns a package that provides low insertion loss and mode free operation at K.sub.a -band frequencies.
2. Description of Related Art
For handling and protection, monolithic microwave integrated circuits (MMIC's), like many other types of integrated circuit dies, are mounted in protective packages that provide shielding and protection for the die during handling and operation. Prior art protective packages include a quartz package and multi-chip MMIC (monolithic microwave integrated circuit) module package.
The quartz packages use fused quartz for the construction of quartz wall and quartz substrate which are the main body of the dielectric layers for the RF transmission lines. The metal conductors are formed on the surface of the quartz substrate through thin film or thick film processing before the quartz wall and the quartz substrate are fired together with glass seal material. The transmission line design of the RF port for the quartz package is based on two sections of microstrip line outside the quartz wall and a strip line section within the wall. The fused quartz material selected for the dielectric layers of the package is good for the RF transmission lines due to the low dielectric constant and the low loss tangent of the quartz material. Unfortunately, with the metal conductors between the quartz wall and the quartz substrate, the quartz seal material for sealing these two layers together do not meet military specifications, such as the MIL-Std-883 seal test requirements. Insertion loss of engineering quartz packages have been tested to be as low as 0.4 dB at 35 GHz from each port (1.24 dB for the two ports), in a model having an integrated microstrip line connecting the two RF input/output ports. This means the testing has not taken into consideration the effect of any wire bonding. Subsequent engineering runs using both discrete microstrip line and wire bonding for connecting the two RF ports have, on the other hand, shown more than 2 dB insertion loss for the two ports.
Analysis of this inconsistent performance has uncovered two major causes of the inconsistency: (1) failure of the glass seal not only causes the package to fail to meet seal test requirements, but also forms inconsistent transmission lines, and (2) wire bonds connecting the package and the discrete microstrip line cause substantial insertion loss. Since both the microstrip line on the RF port of the package and the discrete microstrip line are 50 ohm transmission lines separated by air, the wire bond for connection produces enough inductance to induce significant insertion loss at K.sub.a -band operation. The multi-chip MMIC module package of the prior art was designed to operate at up to 30 GHz. This package was formed of a multi-layer (5 layers) ceramic frame whose layers are metallized over most of both surfaces and co-fired. Additional metallization of the inner wall and outer wall are needed to form an electrical cavity for this kind of ceramic package. The transmission line design of the multi-chip MMIC module package for the RF port is based on microstrip lines outside the wall and a through-wall strip line within the shielded wall. The insertion loss for each of these ports is less than 0.5 dB at 30 GHz. At 35 GHz, the total insertion loss for the two RF ports is expected to exceed 2 dB. Both the above mentioned quartz package and the multi-chip ceramic package have a specific unique configuration to serve a certain purpose. Yet, none of these packages is useful at 35 GHz and beyond. In addition, due to the complexity of the prior art, low cost and commercialization are not easy to achieve.
Accordingly, it is an object of the present invention to provide a K.sub.a -band MMIC package that avoids or minimizes above-mentioned problems.